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“The separation thing is getting really old,” Castle tells his mother this week. And that’s a pretty meta statement, even for a show this self-referential. The estranged Mrs. Castle isn’t even in the episode; the only sign of Beckett in “Cool Boys” is her empty desk and her husband’s continued moping. With the dynamic duo still rent apart by a thin plot device, the position of Castle’s partner is up for grabs weekly. And who should be the lucky contestant this time around but loose cannon detective Ethan Slaughter, played by Nathan Fillion’s Firefly crewmate, Adam Baldwin? Shiny.

Slaughter last appeared in the season 4 episode “Headhunters,” where he wore a fetching brown coat and the writers had a field day with all kinds of references to Joss Whedon’s cult-classic space Western. The in-jokes aren’t as prevalent this time around, but the episode does rely heavily on Fillion and Baldwin’s push-pull chemistry, still as fun as it was on the deck of Serenity.

Slaughter seeks his old collaborator out at Richard Castle Investigations, which registers a “DEFCON One” on Castle and Alexis’s coded alarm system. The detective is working a case that has something to do with the high-profile burglary of a seemingly impenetrable office building, Booth Tower. Castle has no interest in the microchip that was stolen, but can’t resist Slaughter’s bait of juicy plot bunnies for his next book. Ignoring Martha and Alexis’s protests that Slaughter will certainly get Castle into more trouble than he usually finds himself, he goes with him to meet his informant.

That informant ends up being this episode’s dead body, and Slaughter is now mourning “the best snitch” he ever had. Victor Lee was a 30-year-old man who was killed in his own apartment the night before by a knife to the neck. Ryan and Esposito see that Slaughter’s was the last number Victor ever dialed and question him about their conversation. Slaughter plays dumb about Victor’s mystery meeting to shake them off. He knows exactly who the man was going to see but prefers to rock the investigation with Castle and Castle only, à la “Butch and Sundance.” (“You know they die at the end of that movie, right?”)

According to Slaughter, Victor hung out with a group of mouthy criminals but was totally straight himself. Their interview with Victor’s ex tells a different story. The informant, who worked a minimum-wage food-service job, told her he was about to come into some cash, though he didn’t say from where. Slaughter may be a nut, but he has his own moral code. He’s hurt to learn that Victor may not have been immune to the promise of a quick score, but he agrees to follow the lead to the next clue.

The lead is a dealer named Trucco who specializes in moving expensive tech. The microchip lifted from Booth Tower was a prototype of a “next-gen cell phone” and worth upwards of a million dollars to a competitor. Slaughter and Castle track Trucco down, and Castle makes Slaughter promise to play nice going forward. For Slaughter, apologizing to Trucco after he slams him against a brick wall is about as nice as he can play. The guy’s got next to nothing anyway. Victor never showed up for their arranged meeting, so Trucco had no product to shop around. He does mention that Victor had a partner; this is news to Slaughter.

Poor Espo and Ryan have spent more time chasing Castle this season than actual criminals. They’ve been relegated to trailing two steps behind their friend, who’s usually doing something he’s absolutely not supposed to be doing. In this case, he’s still riding with Slaughter after the lab finds his fingerprint on the hilt of the knife that killed Victor. More damning still: Slaughter’s stepped into some hot water in his own precinct and is looking at a possible dismissal. Could his moral code have shifted enough to include conspiring with one of his low-life informants to make some cash?

Castle doesn’t think so, but he understands why, under the circumstances, Slaughter would want to clear his own name of the crime. Ryan and Espo posit another possible scenario: Say Victor’s mystery partner killed him and then realized that his victim had hid the goods before he died; if that partner were Slaughter, he might enlist his “Sherlock” to help him find the chip. “Good thing Castle’s not dumb enough to fall for that,” are famous last words.

All these fools could use a little help in the field, but unfortunately, Hayley Shipton has been offered the recurring position of showing up for just long enough to feed Castle useful information. I can’t imagine that she makes more “consulting” for Richard Castle Investigations than she does running down identity and art thieves for big-time insurance companies. Maybe she just likes the office’s sleek decor. Regardless of whether it’s that or hanging out with Alexis that keeps her there, Hayley digs until she finds a strange and relevant detail about Victor’s personal life. He was a mentor to a troubled teen, and that teen has a genius-level IQ.

Louis Prince, our “criminal Good Will Hunting,” has deep-seated trust issues, or so says the administrator of Operation Mentor. Mr. West tells Slaughter and Castle that Louis rejected five other mentors before clicking immediately with Victor. If Victor had brought Louis in to be the brains of the heist and then betrayed him, that might have been one abandonment too many.